Known fish cages tend to be expensive, relatively complex assemblies that are difficult to displace and offer little or no control in the ascent or descent rate of the cage. The lack of control in the ascent/descent rate of the cage can be quite harmful and even deadly to fish species sensitive to rapid changes of pressure and/or temperature.
An offshore fish farming apparatus, including a semi-submersible platform with a series of silos to hold fish, is disclosed in Canadian Patent No. 1,337,894 to Langlie. The platform includes openings into which the fish silos can be placed. The height adjustment of the fish silos can be controlled by a lifting device disposed on the platform and connected to the silos. The height adjustment of the silos can further be controlled by controlling the depth of the platform by flooding or pumping out buoyancy columns equipped with bilge pumps. This patent does not address the issue of the importance of the ascent/descent rate of the fish silos.
Another apparatus used in aquaculture is disclosed in U.S. Pat. No. 4,257,350 to Steichenberger. The apparatus disclosed therein is that of a fish pen or series of fish pens that can be partially or completely submerged. It also describes the use of constant-buoyancy tanks and variable-buoyancy tanks, the latter adapted to be filled with either air or water. However, as in the Langlie patent, this patent does not address the issue of the importance of the ascent/descent rate of the fish pen.
Yet another apparatus for fish farming is disclosed in U.S. Patent Application Publication No. 2004/0123809 to Klein relating to a fish-breeding apparatus, which comprises a frame supporting one or a plurality of breeding cages. The apparatus is provided with hollow buoyancy elements and means for controlling the buoyancy of the apparatus by filling, totally or partially, the buoyancy elements with water and emptying them of water, totally or partially, whenever desired. The ascent/descent rate of the Klein fish-breeding apparatus is controllable but complicated in that anchors, cables and pulleys are required.
Therefore, it desirable to provide a variable buoyancy fish pen that is easy to assemble, displace and operate while offering improved control in its ascent/descent rate.